The invention relates to a noncontacting displacement measuring system, comprising a sensor with a measuring side and a connection side, an electronic supply/evaluation unit, and a cable leading from the sensor to the electronic supply/evaluation unit and having preferably two inner conductors, with the sensor comprising a housing, at least one coil arranged in the housing, connecting lines leading from the inner conductors of the cable to the coil or respectively coils, and, if need be, an embedding substance which anchors the coil or respectively coils and the connection lines.
For years, noncontacting displacement measuring systems of different types have been known from the practice. They can be classified by their basic mode of operation, first, into displacement measuring systems on the basis of eddy currents, and inductive and capacitative displacement measuring systems, and, second, into optical or acoustical displacement measuring systems.
The present invention relates to a noncontacting displacement measuring system comprising a sensor with at least one coil, i.e., displacement measuring systems, which operate either on eddy-current basis or by induction.
In displacement measuring systems operating by the eddy-current measuring method, a high-frequency alternating current flows through a coil normally cast into a housing, which forms an oscillating circuit by the parallel connection of capacitances. In this process, an electromagnetic field emanates from the coil. This field induces eddy currents in a conductive object, which withdraw energy from the oscillating circuit. Primarily at higher operating frequencies, a reaction of the induced eddy currents appears, which changes as a back induction the real part of the impedance of the coil. The influence on the imaginary part of the coil impedance is, in this process, dependent on the magnetic characteristics and the operating frequency. Nonmagnetic objects of measurement reduce the inductance of the coil when approaching the latter.
The amplitude of the oscillating circuit changes as a function of the spacing. Demodulated, linearized, and amplified, if need be, the change in amplitude supplies a voltage which varies proportionally to the spacing between the sensor and the object of measurement.
In the case of the inductive measuring method, the coil arranged in the sensor is likewise a part of an oscillating circuit. When a conductive object of measurement is approached, the imaginary part of the coil impedance will change primarily. This applies mainly to low operating frequencies, i.e, operating frequencies of up to several 100 kHz. Magnetic objects increase the inductance as they approach the coil, nonmagnetic objects lessen it. Also here, a demodulated output signal is proportional to the distance between the sensor and the object of measurement.
Both in the case of the eddy-current measuring method and in the inductive measuring method, the change in the impedance of a measuring coil arranged in a sensor is measured, when an electrically and/or magnetically conductive object of measurement is approached. The measuring signal thus corresponds to the distance of measurement.
The change in impedance which is create by varying the spacing between the sensor and the object to be measured, thus, results on the one hand from a change in inductance of the coil, and on the other hand from the change in real resistance of the coil. The imaginary part of the coil impedance is thus predetermined, among other things, by the self-capacitance of the measuring coil and, thus, by the entire configuration of the sensor. The electric field lines exiting from the sensor during a measurement, are accordingly also decisive for the self-capacitance of the sensor. When an electrically conductive object of measurement approaches the sensor, the electric field emanating from the sensor is thereby also influenced. This applies likewise, when an object with a relatively high dielectric constant approaches. Consequently, a substance with a high dielectric constant located between the sensor and the actual object of measurement causes a change in the self-capacitance and thus in the total capacitance of the measuring coil.
For example, when water (.epsilon.r .apprxeq. 80) enters between the sensor and the object of measurement, the self-capacitance of the measuring coil will be influenced. In the case of conventional sensors, the change in capacitance amounts to few pF. Should water or another substance with a high dielectric constant be continuously present, it will be possible to consider the influence of the water on the self- capacitance of the coil when calibrating the measuring system. However, if the space between the sensor and the object of measurement is, for example, not splash-proof, that is, should water enter uncontrolled and only temporarily into the range of measurement, errors in the distance measurement will occur, which increase along with the distance between the sensor and the object of measurement. The reason for this is that a movement of the object of measurement effects only a slight change in impedance of the coil at a great distance between the sensor and the object of measurement.
The operating frequency or respectively resonant frequency is calculated with the known formula ##EQU1## wherein C.sub.Spule is the self-capacitance of the sensor coil and C.sub.Erg the supplemental capacitance for the desired operating frequency.
At low frequencies and a given inductance L, the percentage change of the resonant frequency is only slight, since the capacitance C.sub.Erg is substantially greater than the self-capacitance of the coil. The percentage change of the resonant frequency, however, increases quadratically with the frequency, since C.sub.Erg decreases correspondingly. This means that the influence at 1 MHz is 100 times as great as at 100 kHz.
It is therefore the object of the invention to provide for a noncontacting displacement measuring system, in which the influence of fluids or solids with a high dielectric constant on the measured values is largely eliminated.
The noncontacting displacement measuring system according to the invention solves the aforesaid problem in that a shield is provided on the measuring side of the sensor, and that on the one hand the shield is at least largely impervious to electric field lines emanating from the coil or coils, and on the other hand at least largely permeable to electromagnetic field lines emanating from the coil or coils.
According to the invention it has been recognized that the influence of fluids or solids with a high dielectric constant located in the range of measurement on the measured values can be eliminated, when the electric field lines which normally emanate from the sensor, are shielded toward the outside, i.e., when the electric field is closed all around. The material which is used to shield the electric field, should not influence or only slightly influence the electromagnetic field necessary for the measurement.
There are various possibilities of advantageously expanding and further developing the teaching of the present invention. In conjunction with the description of the preferred embodiment of the invention with reference to the drawing, also generally preferred embodiments and further developments of the teaching will be described. Illustrated in the drawing is in